Photoshop "bug" with levels dialog--advice please

I seem to have an incredible (literally "cannot be believed") ability to find obscure bugs, and here is my latest. Any insight is appreciated.

On my main machine (iMac 5K) there is a specific file and a specific set of commands I can execute on that file, that if then choose "Image > Adjustments > Levels" on the mask of a certain layer, a Levels dialog comes up that is completely unresponsive. The dialog does not accept clicks on any part of it, including trying to dismiss it, nor do the carriage return or escape keys help. I have to force-quit Photoshop.

Here is the strange part: I see the same behavior in PS CC 2018, PS CC 2017, and PS CC 2015.5 on this machine. But if I move the file to another machine (a laptop), I have no problems, and I can change the levels. I have never seen behavior like this in years of editing files on this iMac.

So, as far as getting my work done, this isn't a problem--I have made the edit I needed to make (on the other machine) and can continue to edit the file.

But, really, WTF??? The same bug for 3 versions of Photoshop on machine 1, and not there on machine 2? (Both running High Sierra 10.13.3 with effectively the same extensions).

Any ideas what I might look at if something like this comes up again? I re-installed Mac OS on my iMac a few months ago, and don't look forward to doing that again.